214 research outputs found
Magnetic wire-based sensors for the micro-rheology of complex fluids
We propose a simple micro-rheology technique to evaluate the viscoelastic
properties of complex fluids. The method is based on the use of magnetic wires
of a few microns in length submitted to a rotational magnetic field. In this
work, the method is implemented on a surfactant wormlike micellar solution that
behaves as an ideal Maxwell fluid. With increasing frequency, the wires undergo
a transition between a steady and a hindered rotation regime. The study shows
that the average rotational velocity and the amplitudes of the oscillations
obey scaling laws with well-defined exponents. From a comparison between model
predictions and experiments, the rheological parameters of the fluid are
determined.Comment: 14 pages 7 figures, accepted in Physical Review
Intracellular micro-rheology probed by micron-sized wires
In the last decade, rapid advances have been made in the field of
micro-rheology of cells and tissues. Given the complexity of living systems,
there is a need for the development of new types of nano- and micron-sized
probes, and in particular of probes with controlled interactions with the
surrounding medium. In the present paper, we evaluate the use of micron-sized
wires as potential probes of the mechanical properties of cells. The wire-based
micro-rheology technique is applied to living cells such as murine fibroblasts
and canine kidney epithelial cells. The mean-squared angular displacement
(MSAD) of wires associated to their rotational dynamics is obtained as a
function of the time using optical microscopy and image processing. It reveals
a Brownian-like diffusive regime where the MSA scale linearly with time and as
the inverse of the cube of the wire length. This scaling suggests that an
effective viscosity of the intracellular medium can be determined, and that in
the range 1 - 10 micrometers it does not depend on the length scale over which
it is measured.Comment: 13 page, 5 figures, accepted at Biomaterial
Linear and nonlinear rheology of wormlike micelles
Several surfactant molecules self-assemble in solution to form long,
cylindrical, flexible wormlike micelles. These micelles can be entangled with
each other leading to viscoelastic phases. The rheological properties of such
phases are very interesting and have been the subject of a large number of
experimental and theoretical studies in recent years. We shall report on our
recent work on the macrorheology, microrheology and nonlinear flow behaviour of
dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium
Tosilate). This system forms elongated micelles and exhibits strong
viscoelasticity at low concentrations ( 0.9 wt%) without the addition of
electrolytes. Microrheology measurements of have been done using
diffusing wave spectroscopy which will be compared with the conventional
frequency sweep measurements done using a cone and plate rheometer. The second
part of the paper deals with the nonlinear rheology where the measured shear
stress is a nonmonotonic function of the shear rate . In
stress-controlled experiments, the shear stress shows a plateau for
larger than some critical strain rate, similar to the earlier
reports on CPyCl/NaSal system. Cates et al have proposed that the plateau is a
signature of mechanical instability in the form of shear bands. We have carried
out extensive experiments under controlled strain rate conditions, to study the
time-dependence of shear stress. The measured time series of shear stress has
been analysed in terms of correlation integrals and Lyapunov exponents to show
unambiguously that the behaviour is typical of low dimensional dynamical
systems.Comment: 15 pages, 10 eps figure
Universal scattering behavior of co-assembled nanoparticle-polymer clusters
Water-soluble clusters made from 7 nm inorganic nanoparticles have been
investigated by small-angle neutron scattering. The internal structure factor
of the clusters was derived and exhibited a universal behavior as evidenced by
a correlation hole at intermediate wave-vectors. Reverse Monte-Carlo
calculations were performed to adjust the data and provided an accurate
description of the clusters in terms of interparticle distance and volume
fraction. Additional parameters influencing the microstructure were also
investigated, including the nature and thickness of the nanoparticle adlayer.Comment: 5 pages, 4 figures, paper published in Physical Review
Phase Behavior of Polyelectrolyte Block Copolymers in Mixed Solvents
We have studied the phase behavior of the poly(n-butyl
acrylate)-b-poly(acrylic acid) block copolymer in a mixture of two miscible
solvents, water and tetrahydrofuran (THF). The techniques used to examine the
different polymers, structures and phases formed in mixed solvents were static
and dynamic light scattering, small-angle neutron scattering, nuclear magnetic
resonance and fluorescence microscopy. By lowering the water/THF mixing ratio
X, the sequence unimers, micron-sized droplets, polymeric micelles was
observed. The transition between unimers and the micron-sized droplets occurred
at X = 0.75, whereas the microstructuration into core-shell polymeric micelles
was effective below X = 0.4. At intermediate mixing ratios, a coexistence
between the micron-sized droplets and the polymeric micelles was observed.
Combining the different aforementioned techniques, it was concluded that the
droplet dispersion resulted from a solvent partitioning that was induced by the
hydrophobic blocks. Comparison of poly(n-butyl acrylate) homopolymers and
poly(n-butyl acrylate)-b-poly(acrylic acid) block copolymers suggested that the
droplets were rich in THF and concentrated in copolymers and that they were
stabilized by the hydrophilic poly(acrylic acid) moieties.Comment: 11 pages, 12 figures, to appear in Macromolecule
Preventing corona effects: multi-phosphonic acid poly(ethylene glycol) copolymers for stable stealth iron oxide nanoparticles
When disperse in biological fluids, engineered nanoparticles are selectively
coated with proteins, resulting in the formation of a protein corona. It is
suggested that the protein corona is critical in regulating the conditions of
entry into the cytoplasm of living cells. Recent reports describe this
phenomenon as ubiquitous and independent of the nature of the particle. For
nanomedicine applications however, there is a need to design advanced and
cost-effective coatings that are resistant to protein adsorption and that
increase the biodistribution in vivo. In this study, phosphonic acid
poly(ethylene glycol) copolymers were synthesized and used to coat iron oxide
particles. The copolymer composition was optimized to provide simple and
scalable protocols as well as long-term stability in culture media. It is shown
that polymers with multiple phosphonic acid functionalities and PEG chains
outperform other types of coating, including ligands, polyelectrolytes and
carboxylic acid functionalized PEG. PEGylated particles exhibit moreover
exceptional low cellular uptake, of the order of 100 femtograms of iron per
cell. The present approach demonstrates that the surface chemistry of
engineered particles is a key parameter in the interactions with cells. It also
opens up new avenues for the efficient functionalization of inorganic surfaces.Comment: 21 page, 7 figures,Biomacromolecules 201
Dynamic Response of Block Copolymer Wormlike Micelles to Shear Flow
The linear and non-linear dynamic response to an oscillatory shear flow of
giant wormlike micelles consisting of Pb-Peo block copolymers is studied by
means of Fourier transform rheology. Experiments are performed in the vicinity
of the isotropic-nematic phase transition concentration, where the location of
isotropic-nematic phase transition lines is determined independently. Strong
shear-thinning behaviour is observed due to critical slowing down of
orientational diffusion as a result of the vicinity of the isotropic- nematic
spinodal. This severe shear-thinning behaviour is shown to result in gradient
shear banding. Time-resolved Small angle neutron scattering experiments are
used to obtain insight in the microscopic phenomena that underly the observed
rheological response. An equation of motion for the order-parameter tensor and
an expression of the stress tensor in terms of the order-parameter tensor are
used to interpret the experimental data, both in the linear and non-linear
regime. Scaling of the dynamic behaviour of the orientational order parameter
and the stress is found when critical slowing down due to the vicinity of the
isotropic-nematic spinodal is accounted for.Comment: Accepted by J. Phys.: Condens. Matter, CODEF II Special Issue. 20
pages, 9 figure
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